1. Technical Field of the Invention
The present invention relates to a wiring substrate having a metal wiring formed on a substrate, a display device including the wiring substrate, a semiconductor chip for use in the wiring substrate, and electronic equipment employing one of the display device and the wiring substrate.
2. Description of the Related Art
A display device such as a liquid-crystal device is widely used as a display in electronic equipment such as a mobile computer, a mobile telephone, a video camera, etc. An EL (Electro-luminescence) device is beginning to be used as one of display units. The liquid-crystal device includes a pair of electrodes, and an electrooptical material interposed between the electrodes. By controlling a voltage applied between the electrodes, the orientation of the liquid crystal is controlled. When the orientation of the liquid crystal is controlled, light transmitted through the liquid crystal is modulated, thereby externally displaying an image of characters, numbers, etc.
The EL device sandwiches an EL light emission layer as an electrooptical device between a pair of electrodes, and controls a voltage applied between the electrodes, thereby controlling a current fed to the EL light emission layer. Light emission from the EL light emission layer is thus controlled, and an image of characters, numbers, etc. is externally displayed.
Each of the liquid-crystal device and the EL device is formed on a substrate that bears a single or a plurality of electrodes that sandwich the liquid crystal or the EL emission layer. In the liquid-crystal device, for example, a pair of substrates that are opposed to each other have electrodes respectively formed thereon. In the EL layer, on the other hand, a pair of electrodes sandwiching an EL emission layer are laminated on a single substrate. In these display devices, a plurality of electrodes are formed within an effective display area on the substrate, and extension wires leading from a plurality of electrodes and metal wirings different from the extension wires are arranged external to the effective display area. The electrodes formed within the effective display area may be fabricated of an oxide such as ITO, or a metal such as an APC alloy or Cr. When the electrode is formed of a metal, the extension wire leading from that electrode is also fabricated of a metal.
In a so-called COG (Chip On Glass) substrate, in which a semiconductor chip is directly mounted on a substrate, metal wirings, different from the extension wire outwardly leading from the effective display area, are input terminals of a semiconductor chip, such as a metal wiring connected to an input bump, and connected to an FPC (Flexible Printed Circuit) leading from an external circuit.
It is conventionally known that an electrically conductive oxide such as an ITO (an Indium Tin Oxide) is used as an electrode material formed on a substrate in the liquid-crystal device and the EL device. It is also known that a metal such as APC or Cr is used as a material of a metal wiring on a substrate. Here, APC refers to an alloy of Ag (silver), Pd (palladium), and Cu (copper).
Although the ITO has been widely used as a material for the electrode, it has a relatively high electrical resistance. Routing a long line of ITO trace on the substrate results in a high electrical resistance, making it impossible to normally operate a driving circuit. Low electrical resistance metal such as APC or Cr is considered as an effective means to resolve this problem. For example, the resistance of ITO is 15 □ per unit area, while the resistance of Cr is 1.5 Ω per unit area, and the resistance of APC is 0.1 Ω per unit area. Forming a wiring pattern on a substrate using such a low electrical resistance metal advantageously restricts electrical resistance of the pattern even if a long wiring pattern is routed on the substrate.
The use of a wiring pattern fabricated of APC or Cr on a substrate is effective in the reduction of electrical resistance. On the other hand, the APC or Cr wiring pattern is subject to corrosion and subsequent damage, and presents difficulty in maintaining the quality of wiring. The inventors of this invention have conducted various tests in an attempt to resolve the corrosion problem, and have reached the following conclusion. When a plurality of metal wirings are arranged side by side, a voltage difference occurs therebetween. In other words, the relationship of cathode and anode occurs between metal wirings, and the metal of the anode side, for example, Ag, dissolves.
In view of the above problem of the conventional wiring substrate, the present invention has been developed. It is an object of the present invention to prevent a metal wiring from being corroded even when a wiring pattern is fabricated of a low electrical resistance metal.